1. Field of the Invention
The present invention relates to processes for manufacturing sodium carbonate and sodium bicarbonate crystals. More particularly, the present invention relates to methods and apparatus for forming sodium carbonate crystals of a desired size, shape and density.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Much of the world's production of soda ash is produced from natural trona deposits. Natural trona ore is a hydrated mixture of sodium carbonate and sodium bicarbonate along with various organic and inorganic impurities. Currently, soda ash is produced from trona by one of two processes (1) the monohydrate process or (2) the sesquicarbonate process.
In the monohydrate process, trona ore is first calcined in a rotary kiln at temperatures of 175 to 200° C./347 to 397° F. This serves to convert bicarbonate to carbonate. Calcining operations at temperatures between 350 to 400° C. also destroy organic impurities present in the ore. Inorganic contaminants are removed from the calcined trona by dissolving the material in water and recrystallizing sodium carbonate from the filtered solution through the use of heat applied to the water. Soluble inorganic impurities, such as sodium carbonate and sodium sulfate, remain in the mother liquor. Insoluble impurities, such as shale and calcium carbonate, are removed by filtration prior to crystallization. The resulting sodium carbonate crystals, in the monohydrate form, are separated by filtration or centrifugation. The monohydrate crystals are then dried and calcined to anhydrous sodium carbonate. The sesquicarbonate process utilizes basically the same unit operations as the monohydrate process. However, the arrangement of these unit operations differs.
In the sesquicarbonate process, trona ore is first dissolved in hot water and the resulting solution filtered to remove insoluble impurities. Organic impurities are then removed by adsorption of the organics on activated carbon. Pure trona (or sesquicarbonate) is then recrystallized from the purified solution by using triple-effect evaporators. A solution of sodium carbonate (to maintain in excess of 10 to 25% excess carbonate) is recycled in the evaporators so as to obtain the sesquicarbonate. Since trona is an incongruently dissolving double salt, sesquicarbonate cannot be formed by cooling. This, once again, leaves soluble inorganic impurities in the mother liquor. The sesquicarbonate crystals are then calcined to produce sodium carbonate.
These processes are described in detail in various U.S. patents. For example, U.S. Pat. No. 3,479,133, issued on Nov. 18, 1969, to F. M. Warzel describes the monohydrate process. U.S. Pat. No. 3,119,655, issued in January of 1964, to Frint et al. describes the sesquicarbonate process. Similarly, U.S. Pat. No. 3,260,567, issued on July of 1966, to Hellmers et al. and U.S. Pat. No. 3,361,540, issued on Jan. 2, 1968, to Peverly et al. teach these sesquicarbonate processes.
Both the monohydrate and sesquicarbonate processes produce sodium carbonate crystals having a density range of 0.95 to 1.25 g/cc. Some applications (those in which the sodium carbonate is to be used in solution form) prefer the use of lower density crystals or higher surface area crystals. U.S. Pat. No. 5,043,149, issued on Aug. 27, 1991, to Frint et al., and assigned to the FMC Corporation, describes a process for the manufacture of such low density soda ash crystals. Sodium carbonate crystals obtained from all of the above process will vary greatly in size distribution. A large variety of commercial products are produced by the above-described processes. Each of the sodium carbonate crystals formed by these various processes were analyzed for the purpose of showing the size distribution of the crystals. The attached Table I shows the size distribution and shape of the various commercial products:
TABLE ICommercial ProductsGeneral ChemicalITOCHU ChemSolution IDFMC 100FMC 160FMC 260RP LiteRP DenseSyntheticFine SyntheticMeOH Feed rateN/AN/AN/AN/AN/AN/AN/ARP feed rateN/AN/AN/AN/AN/AN/AN/AMeOH feed rateN/AN/AN/AN/AN/AN/AN/AInitial soln. volumeN/AN/AN/AN/AN/AN/AN/ARPMLocation of additionN/AN/AN/AN/AN/AN/AN/ACrystal Density (lb/ft3)49.461.965.315261.43560.2Size Distribution (%)1000 u0.2 850 u0002.80.303.6 600 u0.038.5 425 u0.9821.1 355 u15.510.111.3828.554.74.1 300 u23.3 250 u30.435.829.3931.733.24.3 213 u24.5 150 u29.4813.5 106 u46.449.217.2132.83135.23.1 75 u7.550.9 63 u7.54.62.714.10.833.1 45 u1 <45 u0.3 38 u0.10.20.10.314.5 <38 u00008.5ScreenedxxxxxxxNot ScreenedxDetergency (%)Absorptivity (%)13.912.52517.8Sulfate (ppm)30040070010003000200600TOC (ppm)5628Crystal MorphologyLarge rodsSmall rodsBlockyMixed ballsMixed ballsSmall snowflakesSmall BallsDateMay 26, 1995Jun. 28, 1995Sep. 1, 1995Jun. 28, 1995Jun. 28, 1995Jun. 28, 1995November 1995
The various sizes, shapes and distributions of crystals are applicable in various processes. For example, a large size distribution can adversely affect the dissolving rates of the sodium carbonate and also can produce undesirable dust (at less than about 60 microns). This poses a problem if the material is to be used in dry processes, such as glass manufacturing. In addition, a wide particle size distribution can cause serious problems in the filtration or centrifugation processes which are used to separate the crystals from the mother liquor. As such, it is desirable to form sodium carbonate crystals which have a size distribution, shape and density which mirrors that of commercial products while producing such products at a relatively low cost.
U.S. Pat. No. 4,584,077, issued on Apr. 22, 1986, to Chlanda et al. describes a process for recovering sodium carbonate from trona and other mixtures of sodium carbonate and sodium bicarbonate. This process includes the steps of: (1) forming an aqueous solution comprising sodium carbonate and sodium bicarbonate; (2) removing a portion of the sodium bicarbonate from the solution so as to form a mother liquor comprising sodium carbonate and a reduced amount of sodium bicarbonate; (3) subjecting the mother liquor to an electrodialytic water splitting by circulating the water liquor through an electrodialytic water splitter to produce a liquid reaction product comprising sodium carbonate substantially free of sodium bicarbonate; and (4) withdrawing the liquid reaction product comprising sodium carbonate substantially free of sodium bicarbonate from the electrodialytic water splitter. In this patent, it was described that the sodium carbonate solution product from the base compartment is fed to a primary absorber wherein a liquid loading substance is absorbed into the sodium carbonate solution. The “liquid loading substance” includes liquids such as ammonia, methanol, ethanol and the like. This is added to the sodium carbonate solution to cause the sodium carbonate to crystallize out as the decahydrate, monohydrate or mixtures thereof. As a reaction product, these can be readily separated from the crystallized sodium carbonate-containing material.
This Chlanda process is an extremely energy inefficient process for producing sodium carbonate from trona. A sodium carbonate solution is produced from an electrodialytic water splitter. Sodium bicarbonate is converted to sodium carbonate prior to reacting with the “liquid loading substance”.
U.S. Pat. No. 6,022,385, issued on Feb. 8, 2000, to the present inventor teaches a method of producing sodium carbonate or bicarbonate from any solution or carbonate mineral, but especially from trona, that comprises the steps of: (1) passing a solution containing calcined trona, a solution of carbonate, or tailing pond water to a precipitator; (2) adding methanol of 30% to 70% by volume to the solution in the precipitator so as to precipitate carbonate from the solution, (3) washing the precipitated carbonate with an alcohol-containing solution, and (4) drying the washed precipitated crystals at low temperatures. Fundamentally, the process of the present invention provides a technique whereby sodium carbonate crystals can be formed of various sizes, shapes, densities and distributions by adjusting various parameters of the process. In particular, such sodium carbonate crystals can be produced from various inputs such as from a calcined-sodium carbonate solution, from tailing pond water, from sesquicarbonate or uncalcined trona, or from various mixtures of carbonates and bicarbonates.
Applications of the currently known processes for producing sodium carbonate crystals encounter problems when adjusting for the various types of sodium carbonate containing solutions. Such input solutions range from relatively pure bicarbonate solution, carbonate-bicarbonate solution, sesquicarbonate solution, calcined trona solution, uncalcined trona solution to tailing pond water. Each type of input solution contains various impurities and compounds which reduce the efficiency of the precipitation processes and affect the purity of the sodium carbonate crystals produced.
For example, when processing trona deposits from waste ponds and tailing ponds, the input solution from the ponds contain mostly carbonate and decahydrated carbonate with water impurities over 200,000 ppm and high levels of organics, hardness, silica and sulfates. In U.S. Pat. No. 6,022,385, issued on Feb. 8, 2000, to the present inventor, the amount of impurities may be minimized in sodium carbonate crystals by making large size crystals. The process disclosed in U.S. Pat. No. 6,022,385 also appears to convert the decahydrate carbonate to a lower hydrate and even to a monohydrate. Thus, when using the process of U.S. Pat. No. 6,022,385, additional heat to dehydrate the decahydrated carbonate is not required, unlike other disclosed processes.
Currently, the methods of precipitating sodium carbonate crystals from the various sources of sodium carbonate solutions requires finding means to reduce the amount of impurities from natural or less-refined sources. One method considered is to freeze the input solution in order to reduce the amount of impurities. For organic impurities, some methods include using organoclay to filter out organic impurities. For the hardness impurities, the removal involves steam stripping. For the silica impurities, alumina is used for the silica removal. Both the silica and hardness removal processes are disclosed in U.S. Pat. No. 6,022,385 to the present inventor. It is also known that hardness impurities may be removed by contacting with fresh water and the solid crystals in the precipitation process. These hardness impurities removed by this procedure include calcium and magnesium compounds. However, calcium and magnesium remain in the solution even after known means to attempt to purify the precipitated crystals.
It is an object of the present invention to provide a method for the manufacture of sodium carbonate or bicarbonate crystals that reduces scaling and impurity levels caused by hardness compounds, such as dolomite and trace shorite found in trona formations.
It is an object of the present invention to provide a method for the manufacture of sodium carbonate or bicarbonate crystals that reduces calcium and magnesium impurities from the input solution.
It is another object of the present invention to provide a method for the manufacture of sodium carbonate or bicarbonate crystals that washes excess methanol from the precipitated crystals so as to allow the use of direct fire dryers.
It is a further object of the present invention to provide a process that improves the overall heat balance.
It is still a further object of the present invention to provide a process for removing methanol from the precipitated crystals before drying.
These and other objects of the present invention will become apparent from a reading of the attached specification and appended claims.